1. Field of the Invention
The present invention relates to an improved method for spectrally sensitizing a photographic light-sensitive emulsion necessary for the production of a photographic light-sensitive material and, more particularly, it relates to an improved method for spectral sensitization which can be achieved by effectively adding a dye to an emulsion.
2. Description of the Prior Art
In the production of photographic light-sensitive materials, it is well-known that spectral sensitization (i.e., expanding the light-sensitive wave-length region of a light-sensitive material to the visible region) is important and is indispensable for color light-sensitive materials.
Generally speaking, the spectrally sensitizing steps lie in dyeing light-sensitive elements dispersed in a photographic light-sensitive emulsion, such as silver halide fine crystals, zinc oxide, cadmium sulfide, titanium oxide fine crystals, etc., organic silver complexes, organic high polymer photoconductors, etc., by adding suitable dyes. In particular, many techniques have been developed with respect to methods for spectrally sensitizing silver halide. In the addition of a spectrally sensitizing dye to a silver halide emulsion, the adsorption state on the surface of silver halide fine crystals changes greatly depending upon the addition conditions, which seriously influences the photographic actions such as fogging action, light sensitivity, spectral sensitivity distribution, desensitizing action, stability of sensitivity, etc. This is described in detail in C. E. K. Mees; The Theory of the Photographic Process, 2nd. Ed., Chapter 12, pp.430 - 500, MacMillan Co. (1954). An important condition of providing a stable spectral sensitization while maintaining high sensitivity is that every molecule of the spectrally sensitizing dye added reaches an adsorption site on the light-sensitive silver halide crystals in a stable manner and is adsorbed there without seriously interacting with a binder and without aggregation and precipitation thereof. Also, it is important that the spectrally sensitizing dye is uniformly adsorbed on every light-sensitive silver halide crystal dispersed in the light-sensitive emulsion.
Many methods are known for adding a spectrally sensitizing dye to a light-sensitive photographic emulsion. One method of adding a sensitizing dye as an aqueous solution whose pH has been adjusted to about 6 to 7.5 with a base is described in Japanese Patent Publication No. 27555/69. Also, a method comprising dissolving a spectrally sensitizing dye in a volatile organic solvent having low solubility in water, adding this solution to a hydrophilic colloid for mixing, and heating the resulting mixture to remove the solvent is described in U.S. Pat. No. 3,469,987. In addition, a method of dispersing a substantially water-insoluble, spectrally sensitizing dye in a water-soluble organic solvent without dissolution is described in Japanese Patent Publication No. 24185/71. Further, in a method well-known in the photographic field, a dye is dissolved in a water-soluble organic solvent (e.g., methanol, ethanol, acetone, methyl cellosolve, etc.), and then added to an emulsion. In some cases, water is partly added to the organic solvent.
Many devices have been suggested as to the apparatus for dispersing a dye. For example, U.S. Pat. No. 3,425,835 describes one such apparatus.
The method described in Japanese Patent Publication No. 24185/71 is economically disadvantageous since dispersion for a long period of time using a ball mill is required.
Also, in the method described in Japanese Patent Publication No. 27555/69, spectrally sensitizing dyes in general tend to be decomposed.
On the other hand, two or more spectrally sensitizing dyes in supersensitization with each other are used for spectrally sensitizing light-sensitive photographic emulsions, particularly color photographic emulsions.
Two or more spectrally sensitizing dyes can be separately added to a light-sensitive emulsion by separately dissolving the sensitizing dyes in different solutions. However, with dyes which supersensitize each other, superior photographic properties can be obtained by adding them as a mixed solution of two or more dyes than those obtained by separately adding them. It is well-known in this field that the light sensitivity tends to increase when the dyes are added as a mixed solution. However, when two or more sensitizing dyes are used as a mixed solution, often the defect occurs in that, upon dissolving in the same solvent, one of the dyes is quite unstable in the solvent and a rapid reduction in concentration during storage occurs.
Also, even when two or more dyes are stable in the same solvent, an undesirably large amount of solvent becomes necessary when at least one dye has a very poor solubility.
On the other hand, the dissolving of two or more dyes in the same solvent simplifies the production equipment. That is, only one vessel is necessary for stocking the solution, and the stocked solution can be fed as such upon addition to a light-sensitive photographic emulsion.